Arguments

the density of shading lines, in lines per inch.
The default value of NULL means that no shading lines are drawn.
A zero value of density means no shading lines whereas
negative values (and NA) suppress shading (and so allow
color filling).

angle

angle (in degrees) of the shading lines.

col

color(s) to fill or shade the rectangle(s) with.
The default NA (or also NULL) means do not fill,
i.e., draw transparent rectangles, unless density is specified.

border

color for rectangle border(s). The default means
par("fg"). Use border = NA to omit borders.
If there are shading
lines, border = TRUE means use the same colour for
the border as for the shading lines.

lty

line type for borders and shading; defaults to "solid".

lwd

line width for borders and shading. Note that the use of
lwd = 0 (as in the examples) is device-dependent.

Details

The positions supplied, i.e., xleft, …,
are relative to the current plotting region. If the x-axis goes from
100 to 200 then xleft must be larger than 100 and xright
must be less than 200. The position vectors will be recycled to the
length of the longest.